1. Field of the Invention
The present invention relates to a device and a method of pattern dimension measurement using an electron beam. More specifically, the present invention relates to a pattern dimension measuring apparatus and a pattern area measuring method capable of measuring an area of a contact hole or the like with high reproducibility and accuracy.
2. Description of the Prior Art
Measurement with a scanning electron microscope has been practiced as a method of measuring a pattern line width. A typical scanning electron microscope is configured to irradiate an electron scanning range with incident electrons in a scanning manner, then to acquire secondary electrons that are emitted from a sample via a scintillator, then to acquire image data by converting the quantity of the acquired electrons into luminance, and then to display the image data on a display device.
When characteristics of a semiconductor device are managed by using such a scanning electron microscope, it is a general practice to check whether or not a line width of a line pattern and an area of a contact hole or the like are formed within design standards. The pattern line width and the area are managed in accordance with the following procedures. After a given range of a pattern formed on a photomask is displayed on a screen, an electron beam is focused and irradiated on a measurement point within the displayed range, and thus a waveform representing luminance distribution is acquired based on secondary electrons that are reflected from the measurement point. Then, the luminance distribution waveform is analyzed to find a pattern edge position and to define a line width.
Meanwhile, an area of a contact hole is calculated based on a value of acquired image data. A judgment is made as to whether or not this line width or the area is within an allowable range of error. The line width and area thus judged are used as criteria for quality judgment of the photomask or as process feedback information for a preprocess.
As described above, the measurement of a line width and an area of a pattern is important for a manufacturing process of a photomask. Therefore, various methods of measuring the line width and the area have been proposed to date.
In general, a position where an inclination of luminance corresponding to the quantity of secondary electrons becomes the largest is defined as an edge position of a pattern. In contrast, Japanese Unexamined Patent Publication No. Hei 5 (1993)-296754 discloses an edge detection method of defining, as an edge position, a position where a secondary electron signal becomes the smallest value.
Meanwhile, Japanese Patent Application Laid-open Publication No. 2003-33845 discloses a method of accurately detecting a position of an alignment mark provided on a wafer by capturing an image of the alignment mark with a CCD (charge-coupled device) camera and by performing edge extraction from the captured image.
As described above, the measurement of the pattern line width with use of a scanning electron microscope generally employs the method of defining, as an edge position, a position where an inclination of luminance becomes the maximum or the method of defining, as an edge position, a position where a secondary electron signal becomes the minimum.
On the other hand, the area of the contact hole or the like has been calculated based on luminance information on pixels constituting a SEM image. Specifically, pixels having greater values or smaller values of luminance than a predetermined reference value are extracted from the SEM image, so that the area is calculated by summing up the number of pixels.
In general, the luminance information in the SEM image varies depending on the material, a film thickness or a pattern shape of a measurement object, and also depending on instrument parameters such as an accelerating voltage of an electron beam or the number of scanning operations. Moreover, the luminance information on each pixel contains noise components. Accordingly, the area may be inaccurate when calculated by counting the pixels extracted from the pixel-based luminance information by use of a predetermined luminance threshold.
For example, FIG. 1 is a view showing a portion L of an edge of a contact hole illustrated on pixel-based coordinates P. Pixels shaded in FIG. 1 show smaller luminance values than a predetermined reference value, and these pixels are used for calculating the area of the contact hole. As shown in FIG. 1, the pixels having a predetermined size cannot indicate the edge precisely. For example, a pixel SP2 in FIG. 1 contains portions inside and outside the contact hole. Accordingly, the area value is inaccurate if the pixel SP2 is included in the area of the contact hole.
Moreover, the luminance information represented by these pixels contains noise components. Accordingly, the luminance information is unstable if a S/N ratio is low, and thereby reproducibility of the area value is poor because the pixels selected near the edge may be different in each measurement. For example, a luminance data value for a pixel NP2 may be greater than a predetermined reference value in a certain measurement while being smaller than the predetermined reference value in another measurement.